1. Field of the Invention
The present invention relates to a recording sheet, and specifically to a cooling device that cools a recording sheet after the recording sheet was thermally fixed with a toner image.
2. Related Art
An electrophotographic printing device performs a developing process for developing visible images using colored particles on a surface of a recording sheet and a fixing process for fixing the visible images onto the surface of the recording sheet. Usually, toner in powder form designed to be suitable for electrophotographic printing devices is used as such colored particles. The toner fuses upon heating and fixes upon cooling. The electrophotographic printing device fixes toner images onto a recording sheet by thermal fusion by utilizing such property of the toner in the fixing process.
One type of conventional fixing device used in such an electrophotographic printing device includes a heat roller and a backup roller pressed against the heat roller. The heat roller and the backup roller are collectively referred to as fixing rollers. The heat roller is for generating heat and includes a metal body, which is a hollow tube formed of aluminum, and a heater housed in the metal body. Usually, a halogen lamp is used as the heater. The backup roller serves as a supporting roller and includes a metal shaft coated with a resilient layer, which deforms when pressed against the heat roller, thereby forming a nip portion.
When a recording sheet with a toner image formed thereon passes through the nip portion between the heat roller and the backup roller, pressure and heat are applied to toner forming the image, fusing and deforming the toner. As a result, the toner is fixed onto the surface of the recording sheet. This fixing method is called a heat roller fixing method. Here, only at least one of the two fixing rollers needs to generate heat.
When the recording sheet passes through the nip portion, the toner image contacts the heat roller. Accordingly, there is a danger that fused toner clings onto the surface of the heat roller, which is called offset phenomenon. If offset phenomenon occurs, toner clinging on the surface of the heat roller may be transferred onto a recording sheet during a subsequent fixing process, adversely affecting the printing result.
In order to overcome such a problem, the metal body of the heat roller is usually coated with a mold-releasing layer formed of fluoric resin, fluorine-containing rubber, or silicon rubber. Especially, the fluoric resin is well known in its excellent performance as a mold-releasing member, and so polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA), and the like are well used. Also, an exfoliation claw is attached to the heat roller for stripping the recording sheet off the heat roller.
The recording sheet and the toner fixed onto the recording sheet have substantially the same temperature as the heat roller when the recording sheet is discharged from the nip portion between the fixing rollers. Thereafter, the recording sheet gets cooled down while being transported through a sheet feed path, and then the recording sheet is discharged onto a sheet stacker. However, if the temperature of the recording sheet has not decreased to a glass transition temperature (Tg) before the recording sheet reaches the sheet stacker, then a toner-stick problem occurs.
That is, the fused toner is not completely fixed at a temperature higher than a glass transition temperature of the toner. If recording sheets are stacked one on the other in this condition, then the unfixed toner on a front surface of a recording sheet will stick to a rear surface of another recording sheet stacked thereon. As a result, unnecessary images may be formed on the rear surface of the adjacent sheet, or a part of image may be lost from the recording sheet, due to transfer of toner from the recording sheet to the adjacent sheet. In this manner, image quality may be degraded.
This problem occurs more likely in a high-speed printing device in which sufficient time is not always secured for allowing the recording sheet to cool down after being discharged before being discharged onto the sheet stacker.
In order to overcome such a tone-stick problem, there has been proposed to cool a recording sheet by providing a fan that blows air to the recording sheet. There has been also proposed to dispose a blower for lowering the internal temperature of the sheet stacker by supplying cool outside air into the sheet stacker. However, the fan and blower are disadvantageous in generating noise.
Japanese Patent-Application Publication No. HEI-4-260065 proposes to cool a recording sheet by brining the recording sheet into contact with a cooling roller on a downstream side of the fixing rollers. A heat pipe is used as the cooling roller. A recording sheet may pass through a nip portion between the cooling roller and a resilient support roller resiliently pressed against the cooling roller. Alternatively, the recording sheet may pass through a nip portion between the cooling roller and a belt in contact with the cooling roller.
Since moisture contained in a recording sheet evaporates when the recording sheet is heated, the humidity in the recording device increases when the fixing operation is performed to thermally fix the toner image. Using the cooling roller maintained at a cool temperature makes easier to effectively cool the recording sheet. However, using the cooling roller in such a high-humid environment causes dew condensation on the surface of the cooling roller. This causes errors in sheet transport operations and printing operations.
In order to prevent dew condensation on the cooling roller, Japanese Patent-Application Publication (Kokai) No. HEI-11-15308 proposes to dispose an duct above a cooling roller for ventilating a printing device by supplying outside air to the cooling roller. However, this necessitates a space for disposing a large duct and also a blower for generating air current through the duct, increasing the size of the printing device.